The subject matter of this invention relates generally to VAR generators and relates more specifically to VAR generators of the type employing fixed capacitors and controlled saturating reactors for complementary VAR generation.
It is known to make VAR generators by connecting a fixed capacitor and a continuously controlled inductor in parallel with each other across two lines of an electrical system to be regulated or compensated by the VAR generator. A suitable control system is provided for providing an output signal to the switch portion of the controlled inductor to establish a current conduction interval for the inductor. The conduction interval constitutes a controlled portion of a predetermined period of time, during which current is allowed to flow, thus generating an inductively reactive current which interacts with the fixed capacitively reactive current to produce a net reactive current which cooperates with the voltage across the lines to produce reactive power. The predetermined interval of time is usually one half cycle of the line voltage. Consequently, on a half cycle by half cycle basis, the conduction interval can be changed to provide differing amounts of reactive power as is determined is necessary by the calculating portion of the system. Systems of the previous type can be found in U.S. Pat. No. 3,936,727 issued Feb. 3, 1976 to F. W. Kelly, Jr. and G. R. E. Laison; and U.S. Pat. No. 3,999,117 issued Dec. 21, 1976 to L. Gyugyi et al. The latter patent is assigned to the assignee of the present invention. A number of considerations are related to the following variables: the inductance of the inductor, the conduction interval, the voltage overswing across the lines to be regulated, and reactive current harmonics. To be more specific, it has been found that if the voltage overswing is relatively large as may be the case where there are large transients in the system, the inductance of the controlled reactor must be chosen low enough so that at near maximum conduction interval the resulting inductive reactive current delivered is sufficient to reduce the voltage overswing to a desired level. This implies a significantly smaller conduction interval near normal voltage, which however introduces significant distortion into the compensating current due to the generation of harmonics including the fifth harmonic, for example. It would be advantageous therefore if a way could be found to utilize the inductive properties of the VAR generator to provide compensating reactive current for a wide range of voltage transients without necessitating narrow conduction intervals at near nominal voltage conditions.